Detection of optical radiation using a photodiode structure

ABSTRACT

The central idea of the present invention is that a readout result of an optical detection unit which is based on accumulating photocharges can be improved when the charge carriers accumulated on a photodiode capacitance can be transferred to a readout capacitance before being read out by a readout unit, and when the state of the readout capacitance can be read out in a non-destructive manner by the readout unit, so that a noise portion in the readout signal can be corrected by reading out the readout capacitance during charge accumulation and again reading out the readout capacitance after the end of charge accumulation. Additionally, it becomes possible by the transfer to the readout capacitance to vary the sensitivity of the optical detection device within broad limits and to record a sequence of successive light pulses, without having to reset a photodiode before recording every single light pulse.

TECHNICAL FIELD

The present invention relates to the detection of optical radiation andan optical receiving device as is exemplarily employed in 3D cameras.

BACKGROUND

There are various applications in which optical radiation is to bedetected. An example of applications of this kind are 3D cameras. Inthis context, CMOS image sensorics offer the possibility of detectingdepth by means of near infrared (NIR) light pulse propagation timemethods in a non-tactile manner. The residual intensity of the laserlight reflected by an object is measured here. Principally, there aretwo possible methods, pulse propagation time methods and methodsincluding modulated light. In combination with very short exposureand/or shutter times or suitable demodulation signals, a measurementprovides a residual quantity of the reflected light which isproportional to the distance of the object. 3D cameras are based on thisprinciple.

Using CMOS technology, cameras including intelligent pixels which,except for standard imaging, exemplarily also determine or even track,using a tracking method, the presence of people using movement may berealized. CMOS cameras may even realize combinations of 2D and 3Dimaging.

Using the method of 3D distance measurement by means of CMOS imagesensors, three-dimensional image scenes can be processed electronicallyin real time. A multitude of applications result from this.Three-dimensional inspection and placement systems, for example,necessitate the highest possible degree of image information forreliable object recognition and classification, as is provided by theadditional depth information of a 3D sensor. Further fields ofapplication are in automotive systems, i.e. in the motor vehicle sector,for monitoring tasks, like, for example, monitoring the interior of amotor vehicle including intelligent airbag triggering, theft protection,lane recognition, accident and/or pre-crash recognition, methods forpedestrian protection and parking aids. Further fields of applicationinclude topography measurements, person recognition and presencesensorics. In particular in airbag control, the camera system forexample has the task of providing reliable distance or spacing data,since the airbag has to be triggered by smaller a force depending on thedistance of the passenger. This is possible using 3D-CMOS image sensorswhich provide depth information for every pixel.

FIG. 8 shows the basic structure of 3D measuring systems. An object 802to be measured is irradiated with pulsed or modulated light 806 by alight source 804, the reflected light 808 being imaged on a pixel array812, like, for example, a CMOS chip, by optics 810. Additionally, thesystem includes control means 814 which is coupled to the light source804 and the pixel array 812 to control system operation when measuring.

In the case of a pulse method, the control means 814 for example drivesthe light source 804 such that the object 802 is irradiated by lightpulses 806 which are synchronized in time with exposure slots of thepixel array 812. 3D-CMOS image sensors for distance and/or depthmeasurement here may exemplarily be based on the functional principle ofan active pixel sensor in which the temporal opening of the exposureslot of a pixel is synchronized with the pulsed triggering of activescene illumination.

In the case of a modulation method, the control means 814 for exampledrives the light source 804 such that the object 802 is irradiated withmodulated light 806, the received signal 808 being demodulated on thereceiver side such that the phase difference between the transmittedsignal and the reflected signal provides information on the distance.

The mode of functioning of the system of FIG. 8 when using pulsedirradiation under temporal synchronization of the exposure slots will bedescribed exemplarily referring to FIG. 9. In FIG. 9, three graphs areshown, of which the top one shows the time wave form of the emittedlight intensity of the light source 804, the time t being plotted inarbitrary units along the horizontal axis and the intensity in arbitraryunits being plotted along the y axis. Below that, FIG. 9 shows twographs representing a time wave form of the received light intensity attwo different pixels, namely pixel 1 and pixel 2, of the pixel array812, wherein again the time t is plotted along the x axis and theintensity is plotted along the y axis. As can be seen, the time rangeillustrated extends over two emitted light pulses 902 and 904. Theexposure slots 906 a, 906 b and 908 a and 908 b are located in timesynchronization to the light pulses 902 and 904, respectively, as isindicated in FIG. 9 by broken likes. As can also be seen from FIG. 9,the reflected light pulses 910 a, 910 b and 912 a, 912 b arrive at thepixels 1 and 2, respectively, at different times and/or with a differenttime offset t_(D1) and t_(D2), respectively, which depends on thedistance of the respective object point to the optics 810 and the pixelarray 812 imaged onto pixel 1 and pixel 2, respectively. Due to thedifferent time offset and, in particular, due to the differently sizedoverlap between the respective exposure slots 906 a-908 b and thereceived reflected light pulse 910 a-912 b, different charge quantitiesQ₁ and Q₂ will result in the pixel structures of pixel 1 and pixel 2,respectively. In particular, the charge Q accumulated at every pixel isproportional to the distance r of the object point to the respectivepixel, i.e. Q˜r/2c, c designating the speed of light.

Different problems of different origins arise in the procedure accordingto FIG. 9. A portion of undesired background light will be detected inconnection with the desired reflected light useful signal. Furthermore,the reflectivity of the scene objects influences the portion of thelight reflected. These factors sometimes corrupt the useful signalconsiderably, depending on the distance of the object, and need to beremoved by measuring the object scenery several times, like, forexample, by an additional measurement extending the exposure time slotand additional imaging with no illumination pulses present. Combiningthese measuring results will then result in a 3D measuring result freefrom reflectivity and background light corruption, wherein reference ismade, for example, here to WO 99/34235 A1 where a system according toFIG. 8 using a CMOS-CCD camera is described.

Another problem is that the propagation time of the pulsed and/ormodulated light 806 is very short and exemplarily is in the range ofnanoseconds, this also being the reason for the fact that the chargequantity Q containing the depth information is small so that thesignal-to-noise ratio as results by evaluating a pulse is relativelygreat. One way of improving the signal-to-noise ratio is accumulatingthe charge quantities of successive pulses on the pixel structure of thecorresponding pixel or in an external circuit to thereby obtain animproved signal-to-noise ratio by signal averaging.

In the pulse method, for example, this way of improving thesignal-to-noise ratio is basically possible, however the pixel has to bereset after each pulse of the sequence. In FIG. 9, a reset processexemplarily takes place at the beginning of the exposure slots 906 a-908b, the reset being when a charge quantity to be read out at the end ofthe exposure slot is set to a predetermined value. However, the resetprocess itself generates a noise contribution to a considerable degreewhich, in turn, corrupts or reduces the signal swing obtained by amultiple pulse sequence.

Since in industry there is great demand for precise 3D measuringsystems, as has been described in the introduction, in industry there isalso demand for an improved detection of electromagnetic radiation forreducing the negative influence caused by reset processes.

In “CCD-Based Range Finding Sensor”, IEEE Transactions on ElectronicDevices, Vol. 44, No. 10, Oct. 1997, pp. 1648-1652, a method formeasuring distances by means of a CCD sensor is described. In thissystem, light pulses which, after reflection at the object, impinge onthe photogate of a pixel are used. Below the photogate, photoelectronsare generated which, depending on the driving of two slot gates to onerespective side of the photogate, are transferred to a first memory gateor a second memory gate. This alternating accumulation at the memorygates is performed over several periods.

WO 02/33817 A1 describes a method and a device for detecting andprocessing signal waves where every pixel has two photodiodes which areprovided with a voltage alternatingly to demodulate modulated lightsignals in a manner phase-offset to each other. A similar procedure isdescribed in WO 04/086087 A1.

SUMMARY

According to an embodiment, an optical detection device may include aphotodiode structure having a photodiode capacitance for accumulatingcharge carriers responsive to electromagnetic radiation, a readoutcapacitance, resetting means for resetting the readout capacitance byproviding the read-out capacitance with a predetermined voltage,switching means for connecting the photodiode structure to the readoutcapacitance to transfer the accumulated charge carriers onto the readoutcapacitance during a transfer phase and for separating the photodiodestructure from the readout capacitance during an accumulation phase,readout means for reading out the readout capacitance, the readout meansbeing implemented to read out the readout capacitance for a first timeduring the accumulation phase and a second time after the accumulationphase for obtaining a first and second readout value, and to combine thetwo values for obtaining a readout result, and an additional readoutcapacitance having respective addition resetting means for resetting theadditional readout capacitance, and additional switching means forconnecting the photodiode capacitance to the additional readoutcapacitance during an additional transfer phase.

According to another embodiment, a system for 3D measurements of objectsmay have: a pulsed light source; a plurality of devices mentioned abovethe detection means of which are arranged in a matrix, wherein theaccumulation phase and the transfer phase are synchronized with aradiation phase by means of the switching means; imaging means forimaging an object onto the matrix; and evaluating means for generating3D information concerning the object based on the states of the readoutcapacitances of the plurality of devices.

According to another embodiment, a method for operating an opticaldetection device including a photodiode structure having a photodiodecapacitance for accumulating charge carriers responsive toelectromagnetic radiation and a readout capacitance, and an additionalreadout capacitance, may have the steps of: resetting the readoutcapacitance by applying a predetermined voltage to the readoutcapacitance by means of resetting means; connecting the photodiodestructure to the readout capacitance by switching means to transfer theaccumulated charge carriers to the readout capacitance during a transferphase and separate the photodiode structure from the readout capacitanceduring an accumulation phase; reading out the readout capacitance byreadout means, the readout means being implemented to read out thereadout capacitance for a first time during the accumulation phase andfor a second time after the accumulation phase for obtaining a first andsecond readout values, and to combine the two values for obtaining areadout result; resetting the additional capacitance; connecting thephotodiode structure to the additional readout capacitance by additionalswitching means to transfer the second accumulated charge carriers tothe additional readout capacitance during a transfer phase and separatethe photodiode structure from the additional readout capacitance duringan accumulation phase; and reading out the additional readoutcapacitance to read out the additional readout capacitance for a firsttime during the accumulation phase and for a second time after theaccumulation phase for obtaining a first and second readout values, andto combine the two value for obtaining an additional readout result.

An embodiment may have a computer program having a program code forperforming the above mentioned method for detecting optical radiation,when the program runs on a computer.

The central idea of embodiments of the present invention is allowing areduction in noise when a photodiode structure having a photodiodecapacitance is assigned a readout capacitance which is separated fromthe photodiode structure by switching means, and when readout means isprovided which, for obtaining a readout result, does not only read out areadout value from the readout capacitance after the accumulation phasebut also once during the accumulation phase to combine the two readoutvalues, like, for example, to calculate the difference, so that thereset noise forming when the readout capacitance is reset can beeliminated from the readout result or reduced.

According to an embodiment of the present invention, the photodiodestructure is formed of a pinned photodiode the space charge zone ofwhich can be depleted, which in particular entails the advantage that,in a pinned photodiode, the light-sensitive p-n junction is not coveredby a metal electrode and is close to the surface so that a pinnedphotodiode has higher a sensitivity.

According to another embodiment of the present invention, the firstreadout of the first readout value takes place directly after resettingthe readout capacitance, the second readout taking place after the endof the accumulation phase or in a subsequent transfer phase. The voltagestate of the readout capacitance can for this purpose be read out at anytime such that the state of the readout capacitance does not changeduring the readout process. Additionally, the read-out voltage state ofthe readout capacitance can be latched in an analog, but also in adigitalized way. The portion of the reset noise in the final readoutresult here is corrected by subtracting the two latched readout valuesfrom each other at the end of a complete readout cycle, so that thecontribution of the reset noise in the final readout result iseliminated or at least reduced.

In another embodiment of the present invention, it becomes possible torecord a sequence of several successive light pulses without having totrigger a reset event of the read capacitance after each light pulse. Atthe beginning of the charge accumulation of a first accumulation phase,the readout capacitance is reset, and directly after that the state ofthe readout capacitance is read out for a first time and stored. Afterthe end of the first charge accumulation phase, the accumulated chargeis transferred from the photodiode to the readout capacitance. Thephotodiode can, without the readout capacitance having to be reset,start a new accumulation cycle, since its space charge zone has beenrestored by the depletion due to the charge transfer. At the end of thefirst accumulation cycle, a transfer of the photocharges onto thereadout capacitance takes place again. This process may in principle berepeated as frequently as desired, wherein it should be kept in mindthat the precharged readout capacitance is continued to be dischargedwith each charge transfer by the accumulated charge in the photodiode,so that the maximum number of accumulation phases to be recorded resultsfrom the magnitude of the readout capacitance and the reset potential.This is referred to as so-called full well capacity. At the end of thelast accumulation phase, the voltage state of the readout capacitance isread out for a second time and also latched, the final readout resultresulting from calculating the difference of the second readout resultand the first readout result and, again, being free from a noiseportion. An optical detection device operated in this way contributes toa significant improvement in the signal-to-noise ratio and can berealized using a standard CMOS structure. Several successiveaccumulation phases will be possible without having to trigger a resetevent after each individual accumulation phase, which would deterioratethe signal-to-noise ratio again due to the unavoidable noisecontribution.

According to another embodiment of the present invention, thecontribution of the background light to the final readout result can becorrected efficiently, in addition to correcting the reset noise. Thisis made possible by the fact that the pinned photodiode can be connectedin a random fashion to three readout capacitances, each readoutcapacitance containing its own reset means and its own readout device.Control means in this embodiment is to be extended in that it canconnect the photodiode to any of the three readout capacitances atfreely selectable times, and that the reset device and the readoutdevice of each of the three readout capacitances can also besynchronized with emitting the light pulse of the pulsed light source.

A first accumulation phase begins after the light pulse has been emittedfrom the light source. After the end of the first accumulation phase,the generated photocharges of the first accumulation phase aretransferred to the first readout capacitance by connecting thephotodiode capacitance to the first readout capacitance, are read outand latched as a first readout result. During a second accumulationphase which is performed without active object illumination, the chargecarriers generated on the photodiode are only generated by thebackground light present. At the end of the second accumulation cycle,the charge accumulation on the photodiode is transferred to the secondreadout capacitance and is subsequently read out, the second readoutresult obtained in this way containing a background light signal. Thebackground light portion in the first readout result is, according tothe embodiment, eliminated in an advantageous manner by subtracting thesecond readout result from the first readout result.

In another embodiment of the present invention, a first photodiodestructure and a second photodiode structure are used for realizing adistance measuring method according to the modulation principle. Thecontrol means here additionally controls the modulation of the emittedlight signal and, by means of a demodulation signal, the transfer timesof the accumulated photocharges of the first photodiode to the firstreadout capacitance and the second photodiode to the second readoutcapacitance. The information on the phase difference and thus thedistance information are contained in the different charge quantitieswhich are read out at the times preset by the demodulation signal. Inthe embodiment a distance measuring method, which is based on themodulation method, is realized completely in a standard CMOS process,the light-sensitive regions not being covered partly by opaquestructures like in the other conventional devices, thereby improvingsensitivity and the signal-to-noise ratio.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will be detailed subsequentlyreferring to the appended drawings, in which:

FIG. 1 is a circuit diagram of an optical detection device according toan embodiment of the present invention;

FIG. 2 is a semi-schematic cross-sectional drawing of a part of thedevice of FIG. 1;

FIG. 3 shows graphs where exemplary time wave forms of signal andswitching voltages as they occur in the device of FIG. 1 are shown;

FIG. 4 is a circuit diagram of an optical detection device according toanother embodiment of the present invention;

FIG. 5 shows graphs where exemplary time wave forms of signal andswitching voltages as they occur in the device of FIG. 4 are shown;

FIG. 6 is a schematic drawing of an optical detection device accordingto another embodiment of the present invention;

FIG. 7 shows graphs where exemplary time wave forms for illustrating themode of functioning of the device of FIG. 6 are shown;

FIG. 8 is a schematic drawing of a 3D measuring system; and

FIG. 9 shows graphs where exemplary wave forms as they occur in ameasuring system of FIG. 8 when using a pulse method are plotted.

DETAILED DESCRIPTION

Before the present invention will be detailed subsequently referring tothe drawings, it is pointed out that in the figures same elements areprovided with same or similar reference numerals and that a repeateddescription of these elements is omitted.

Also, it is pointed out that, in the subsequent description of thefigures, it is assumed that the optical detection devices shown are partof the system of FIG. 8, which is why reference is made partly to FIG. 8and the elements thereof without describing the structure of FIG. 8again. In particular, the optical detection devices described below may,for example, represent one single pixel of the pixel array 812, as willbecome clear from the subsequent description.

A first embodiment of an optical detection device will be describedreferring to FIGS. 1-3. FIG. 1 shows the structure of the opticaldetection device which in FIG. 1 is generally indicated by 10. Theoptical detection device 10 includes a pinned photodiode 12, a transferswitch 14, a readout capacitance 16, a reset switch 18 and a readoutcircuit 20. The pinned photodiode 12 includes two p-n junctions which inFIG. 1 are illustrated as a photodiode 22 and a photodiode capacitance24. The photodiode 22 and the capacitance 24 are connected, in parallelto each other, between ground and the switch 14 which, in turn, isconnected between the pinned photodiode 12 and a circuit node 26. Thecircuit node 26 is connected to the input of the readout circuit 20. Thereset switch 18 is connected between the circuit node 26 and a supplyvoltage terminal 28 to which a supply voltage U_(DD) is applied. Thereadout capacitance 16 is connected between ground and the circuit node26.

The readout circuit 20 in turn includes a unity-gain amplifier 30 and aCDS (correlated double sampling) stage 32 which are connected in seriesbetween an input of the readout circuit 20 and an output of the readoutcircuit 20, the output of the readout circuit 20 at the same timerepresenting an output of the optical detection device 10 where thereadout result and/or readout voltage U_(out,CDS) is output.

The CDS stage includes an output-side difference calculator 34, a seriesconnection of a selection switch 36 a and a unity-gain amplifier 38 aand 36 b, respectively, and 38 b being connected between an input of theCDS stage and the positive and negative inputs of the differencecalculator 34. A latch capacitance 40 a and 40 b is connected to groundat a circuit node between the selection switch 36 a, b and an input ofthe unity-gain amplifier 38 a, b.

FIG. 2 shows an embodiment of a possible implementation of the device ofFIG. 1, the readout circuit 20 being only illustrated schematically as ablock.

According to this implementation, the device of FIG. 1 is formed in ap⁻-epi layer or lightly doped epitaxial p-type layer 50 which isarranged on a p⁺-type or highly doped p-type substrate 52. As can beseen, the pinned photodiode 12 is formed of an n-doped well 54 in thelayer 50 at the exposed side of which, in turn, a p⁺-doped region 56 isarranged so that a p-n junction 58 representing the photodiode 22 ofFIG. 1 is formed in the proximity to the exposed surface. The p-njunction 60 between the n-well 54 and the p⁻-type layer 50 forms thephotodiode capacitance 24, wherein, although not shown in FIG. 2, thesubstrate 52 and the layer 50 are connected to ground. On the exposedsurface of the layer 50, two further n⁻-doped regions 62 and 64 areformed, wherein the first one mentioned, in combination with the n-well54, forms a transistor or MOS transistor representing the switch 14 andadditionally comprising a gate electrode 66 extending above the n-well54 and the n⁻-type region 62 and between same and being separated fromthe n-well 54 and the n⁻-type region 62 by a silicon dioxide layer 68.Similarly, a layer arrangement of a silicon dioxide layer 70 and a gateelectrode 72 extends between the n⁻-type region 62 and the n⁻-typeregion 64 to form a transistor or MOS transistor acting as a resetswitch 18. As is shown in FIG. 2, the p-n junction between the n⁻-typeregion 62 and the p⁻-type layer 50 forms the readout capacitance 16.

As will be discussed below in greater detail referring to FIG. 3, it ismade possible by suitably controlling the switches 14, 18, 36 a and 36 bto achieve, by means of the pinned photodiode 12, readout which isbasically free from reset noise which occurs unavoidably when resettingthe readout capacitance 16, as will be discussed in greater detailbelow. In particular, the switch 14 is controlled by a control signal TGapplied to the gate electrode 66, the reset switch 18 controlled by areset signal Reset applied to the gate electrode 72, the selectionswitch 36 a controlled by a signal Select1 a and the switch 36 bcontrolled by a signal Select1 b. These signals are, for example,generated by the control means 814 (FIG. 8), wherein in this case thepinned photodiode 12 exemplarily represents a pixel of the pixel array812.

The mode of functioning of the device of FIG. 1 will be discussed belowreferring to FIG. 3, wherein at the same time reference is made to FIGS.1 and 2 and, where appropriate, also to FIG. 8.

FIG. 3 shows a series of signals, time being plotted on the x axis. Thetop illustration shows the time wave form of the sending process of alight pulse 806 sent by the light source 804. Directly below that is thetime wave form of the intensity of the reflected light pulse incident onthe photodiode 22, the light pulse arriving at the sensor 812 offset bythe light propagation time T_(L). The next thing shown is the signalReset by means of which closing the switch 18 of FIG. 1 is controlled.What follows is the representation of the signal Select1 a foropening/closing the switch 36 a, followed by the signal TG foropening/closing the switch 14. The next thing shown below is therepresentation of the voltage form U_(FD) across the readout capacitance16 and the voltage form U_(PD) as results across the photodiodecapacitance 24. The signal Select1 b controls closing the switch 36.

Before image recording, the n-well 54 of the pinned photodiode 12 has tobe depleted so that the so-called “pin potential” U_(PD)=U_(pin) willform at the p-n junction 60 across the junction capacitance. At first,the readout capacitance 16 is brought to the potential of the supplyvoltage U_(DD) by means of the reset signal 80. After that, the actualdepletion of the n-well 54 from the photodiode 12 is controlled by thesignal 82, whereby switch 14 is closed and charges still stored on thephotodiode capacitance 24 dissipate to the readout capacitance 16 at thepotential U_(PD). With the falling edge of signal 82, at the time t₁, aphase 88 of charge accumulation begins where the photo-generated chargecarriers are accumulated in the space charge zone of the pinnedphotodiode 12 and the potential U_(PD) at the photodiode capacitance 24decreases proportionally from the starting potential U_(pin) to thelight quantity detected. At the time t₁, simultaneously to the fallingedge of the signal 82, the readout capacitance is reset again by thereset signal 84, wherein, after the falling edge of the signal 84, avoltage U_(FD) made up of the supply voltage U_(DD) and a noise voltageportion u_(n1) is applied across the readout capacitance 16, the resultbeing U_(FD)=U_(DD)+u_(n1).

After the end of the reset pulse 84, the switch 36 a is closed by thesignal 86 and the voltage state of the readout capacitance 16 is readout in a non-destructive manner via the unity-gain amplifier 30 of FIG.1 and latched in an analog manner by the capacitance 40 a and therespective unity-gain amplifier 38 a. The reset noise contribution isread out and stored by resetting by the signal 84 and subsequentlyreading out the readout capacitance 16 by the signal 86 during the phase88 of charge accumulation of the pinned photodiode 12. The phase 88 ofcharge accumulation of the pinned photodiode 12 includes a first phase90 when the reflected light signal has not yet reached the sensor 812due to its finite light propagation time T_(L) so that only the naturalbackground light intensity E_(H) is detected by the pinned photodiode12, and thus the potential U_(PD) across the photodiode capacitance 24only decreases proportionally to E_(H) during the phase 90. After thelight propagation time T_(L), the reflected light reaches the photodiode12 so that, up to the end of charge accumulation at the time t₃ during aphase 92 of charge accumulation, the potential U_(PD) across thephotodiode capacitance 24 decreases faster than during the phase 90,since the photodiode 12 now detects the sum of the background lightintensity E_(H) and the laser light intensity E_(L). The influence ofthe background light on the readout result can be corrected according toanother embodiment of the present invention of FIG. 4, which will bediscussed there in greater detail.

At the end of the accumulation phase 88 of the photodiode 12, at thetime t₃, the accumulated photocharge Q_(PD) is transferred to thereadout capacitance 16 by the signal 94. During the duration of thetransfer phase determined by the signal 94, free photocharge carrierscontinue to be generated by the photodiode 12, however they dissipatedirectly to the readout capacitance 16 so that during the transfer phasethe voltage U_(PD) at the photodiode capacitance 24 does not decrease,as is the case during the accumulation phase 88. Due to the fact thatthe reset noise portion is still resident on the readout capacitance 16,the signal is corrupted by this contribution so that the voltageU_(FD)=U_(DD)+u_(n1)−(Q_(PD)/C_(FD)) is applied to the readoutcapacitance 16 after the end of the charge transfer at the time t₄.After the end of the charge transfer phase controlled by a signal 94,the voltage U_(FD) at the readout capacitance 16 is read out via theunity-gain amplifier 30 in a manner controlled by a signal 96 andlatched in an analog manner on the capacitance 40 b and the respectiveunity-gain amplifier 38 b. The voltage at the output of the differencecalculator 34 will then be U_(out,CDS)=Q_(PD)/C_(FD), which means thatthe reset noise portion in the signal at the output of the differencecalculator is eliminated.

An advantage of the embodiment of FIGS. 1-3 is that a complete sequenceof light pulses can be recorded in succession by means of the detectiondevice 10 without having to reset the readout capacitance 16 again aftereach light pulse. This causes a significant improvement in thesignal-to-noise ratio of the readout value stored on the latchcapacitance 40 b, since a reset noise contribution only occurs onesingle time for a repeated charge accumulation, i.e. signalmultiplication.

Another accumulation phase of the pinned photodiode in FIG. 3 couldbegin with the falling edge of the signal 94 at the time t₄ when theswitch 14 is opened again and the accumulated photocharge carriersreduce the voltage U_(PD) across the photocapacitance 24 again, as hasoccurred in analogy during the first accumulation phase 88. Thus, nsuccessive light pulses can be recorded by the detection device 10 withonly one single reset process so that the signal-to-noise ratio can beimproved significantly by multiple pulse recording.

Recording laser light can thus be performed for one single or for manysuccessive light pulses by means of the detection device 10. Insingle-pulse operation, the photodiode 12 and the readout capacitanceand/or the floating-diffusion capacitance 16 are usually reset by meansof the reset switch 18 before each new pulse and the noise in the CDSstage 32 is corrected. In multi-pulse operation, the photocharge ofsuccessive light pulses is accumulated in the readout capacitance 16.

In the detection device 10, the magnitude of the output voltage at thereadout capacitance 16 may be selected freely within broad limits independence on the ratio of the photodiode capacitance 24 and the readoutcapacitance 16, i.e. the sensitivity of the detection device 10 is notsolely preset by the geometrical extension of the pinned photodiode 12.For clarification purposes, FIG. 2 shows a potential form 100 as formswithin the detection device. A potential well 102 depending on thesupply voltage U_(DD) forms below the supply voltage terminal 64.Directly after the reset process 80, the supply voltage U_(DD) is alsoacross the readout capacitance 16 so that a potential well 104 below then-well 62 representing the readout capacitance 16 at this time has thesame depth as the potential well 102. The depth of a potential well 106below the pinned photodiode 12 is determined by the voltage U_(pin)settling across the depleted p-n junction 60.

The magnitude of the photodiode capacitance 24 across the p-n junction60 is determined by the doping of the semiconductor material used andadditionally depends on the geometrical extension of the n-doped well54. The photocharges 108 generated (Q_(PD)) are accumulated in thepotential well 106 below the photodiode 12, the depth of the potentialwell 106 decreasing with an increasing number of photocharges. Duringthe transfer phase, the accumulated charges Q_(PD) are transferred tothe readout capacitance 16, i.e. accumulated in the potential well 104,where they are added to charges 110 (Q_(FD)) which may already bepresent in the potential well 104. Assuming ΔU_(PD)=Q_(PD)/C_(PD), thefollowing results: ΔU_(FD)=(C_(PD)/C_(FD))*ΔU_(PD). The signal swingΔU_(PD) across the photodiode 12 thus is present at the output 30amplified by the factor (C_(PD)/C_(FD)). A voltage swingΔU_(FD)=Q_(PD)/C_(FD) per transfer process is achieved. The signal swingΔU_(FD) of the readout signal and thus the sensitivity of the receivingdevice 10 may thus be varied, with a predetermined geometry of thephotodiode 12, by selecting the readout capacitance C_(FD), wherein acompromise has to be made between clock punch-through and amplificationachievable.

In single-pulse operation, the readout capacitance 16 is reset and thenoise contribution read out before charge accumulation. In single-pulseoperation, the highest signal amplification possible is desirable, sothat in this case the readout capacitance 16 (C_(FD)) would be selectedto be small compared to the photodiode capacitance 24 (C_(PD)), whereinexemplarily C_(PD)=7 pF and C_(FD)=1 pF are possible values. Inaddition, single-pulse recording may be repeated as often as desired,wherein the readout capacitance is reset and the noise contribution isread out between successive light pulses so that the noise contributionin the final readout result is compensated completely and is independentof the pulse number N selected.

When using the detection device 10 as a single light-sensitive pixel ofa two-dimensional pixel array in the sensor 812, for each pixel, thenoise voltage portion read out by means of the signal 86 and the noisevoltage portion read out by the signal 96 and depending on thephotocharges generated have to be stored individually for each pixel inan analog manner, like, for example, “on-chip”, i.e. on the sensor. Forsmall numbers of pixels, this may still be realized, but no longer inlarge pixel matrices. With large pixel matrices, the noise contributionconsequently is not read out before, but instead N light pulses arerecorded in sequence and transferred to the readout capacitance 16 sothat the noise error occurs once in the entire readout result. Here, thenoise contribution is not read out before, but occurs once,independently of N, in the readout result. The readout capacitance 16 inthis latter case would be selected to be relatively large in relation tothe photodiode capacitance 24 so that a small number of charge transfersof the photodiode capacitance 24 to the readout capacitance 16 does notalready decrease the depth of the potential well 104 so strongly thatthe mode of functioning of the detection device of FIG. 2 is no longerguaranteed. Typical values of a corresponding usage of the detectiondevice would, for example, be C_(PD)=7 pF and C_(FD)=3-5 pF.High-resolution distance detector matrices can consequently be realizedby this method, which has not been possible so far due to too muchnoise.

The problem of interfering noise caused by resetting is solved and/orreduced strongly in all methods by the detection device, the voltageswing and/or the charge amplification at the readout capacitance 16(C_(FD)) is proportional to the ratio C_(PD)/C_(FD).

According to another embodiment of the present invention shown in FIG.4, correction of the undesired background light portion in the readoutsignal can be performed efficiently. Extending the detection device ofFIG. 1, a way of connecting the pinned photodiode to three circuit nodesand/or floating-diffusion nodes and thus to three output capacitances isprovided, wherein this embodiment may be extended nearly arbitrarily.

FIG. 4 shows the pinned photodiode 120 the photo-sensitive p-n junction122 and the photodiode capacitance 124 of which are connected betweenground and a circuit node 126. Additionally, the circuit includes three,on the circuit side, identical readout branches 128 a, 128 b and 128 cand three switching means 130 a, 130 b and 130 c. The readout branch 134a includes a readout capacitance 132 a, a reset switch 134 a, aunity-gain amplifier 136 a, a readout selection switch 138 a and a CDSstage 140 a. The readout branch 128 b includes a readout capacitance 132b, a reset switch 134 b, a unity-gain amplifier 136 b, a selectionswitch 138 b and a CDS stage 140 b. The readout branch 128 c includes areadout capacitance 132 c, a reset switch 134 c, a unity-gain amplifier136 c, a readout selection switch 138 c and a CDS stage 140 c. The CDSstages are not illustrated in detail, but only schematically.

The circuit node 126 is connected to the circuit node 142 a of the firstreadout branch 128 a via switch 130 a, is connected to the circuit node142 b of the second readout branch 128 b via switch 130 b and isconnected to circuit node 142 c of the third readout branch 128 c viaswitch 130 c. The circuit node 142 a can be connected to a supplyvoltage U_(DD) not shown via switch 134 a. The readout capacitance 132 ais connected between ground and the circuit node 142 a of the firstreadout branch, the circuit node 142 a being further connected to theinput of the unity-gain amplifier 136 a. The output of the unity-gainamplifier 136 a may be connected to the input of the CDS stage 140 a(CDS1) illustrated here only schematically by means of switch 138 a. Thecircuit node 142 b of the second readout branch can be connected to thesupply voltage U_(DD) via switch 134 b. The readout capacitance 132 b isconnected between the circuit node 142 b and ground. The input of theunity-gain amplifier 136 b is connected to the circuit node 142 b, theoutput of the unity-gain amplifier 136 b being connected to the input ofthe CDS stage 140 b (CDS2) via switch 138 b. The circuit node 142 c ofthe third readout branch can be connected to the supply voltage U_(DD)via switch 134 c. The readout capacitance 132 c is connected between thecircuit node 142 c and ground. The input of the unity-gain amplifier 136c is connected to the circuit node 142 c, wherein the output of theunity-gain amplifier 136 c can be connected to the input of theschematically illustrated CDS stage 140 c (CDS3) via the switch 138 c.

Using the embodiment of the present invention shown in FIG. 4, thesignal accumulated by a pinned photodiode 126 can be evaluated such thateffects resulting from the influence of background light and reset noisecan be eliminated. Here, the photocharge Q_(PH) proportional to thelight may be shifted from the photodiode 120 to the readout capacitances132 a and 132 b at different times such that the background lightportion can be separated from the laser light portion. The third readoutbranch 128 c via the switch 130 c here only serves for asynchronouslysetting the photodiode 120 at any time, which is necessary to be able toerase dark currents and light incident on the photodiode 120 before theactual laser signal recording. It is of advantage for the reset switch134 c of the third readout branch 128 c to remain closed all the timebefore and after the acquisition phase. The depletion of the photodiode120 takes place exclusively by closing switch 130 c, preventing anon-correctable reset noise portion from remaining on the photodiodecapacitance 124, as would be the case if at first switch 130 c and afterthat switch 134 c were closed for the purpose of resetting thephotodiode 120. Consequently, the first-mentioned switching order is tobe used.

The mode of functioning of the embodiment of FIG. 4 will be explainedbelow referring to FIG. 5, wherein the detection device of FIG. 4 isexemplarily operated as a pixel of the light-sensitive sensor 812 fordepth measurements, wherein both the contribution of the backgroundlight and the contribution of the reset noise in the readout result areto be corrected. Synchronizing the emission of the light pulse 806 ofthe light source 804 at the actuating times of the switches 130 a, b, c134 a, b, c and 138 a, b, c by means of suitable signals here isperformed by the control means 814. In FIG. 5, time is plotted on the xaxis measured in units T, the duration of a complete accumulation cycle,which includes a charge accumulation phase including active sceneillumination and a charge accumulation phase without active sceneillumination. The signals Reset1 for actuating switch 134 a, Select1 foractuating switch 138 a, TG1 for actuating switch 130 a, TG2 foractuating switch 130 b, Reset2 for actuating switch 134 b, Select2 foractuating switch 138 b and TG3 for opening and closing switch 130 c aregenerated by the control means 814.

What is also illustrated is the voltage U_(FD,1) across the readoutcapacitance 132 a of the first readout branch 128 a, the voltageU_(FD,2) across the readout capacitance 132 b of the second readoutbranch 128 b and the voltage U_(PD) across the photodiode capacitance124. Generally, the following applies for the node voltage U_(PD) andthe node voltages U_(FD) across the readout capacitances 132 a, b or c(C_(FD)) during the charge accumulation and/or transfer phase, where oneof the switches 130 a, b or c is closed:

$\begin{matrix}{U_{PD} = {{\frac{Q_{{PD}\;}}{C_{PD}}\bigwedge U_{FD}} = \frac{Q_{PD}}{C_{FD}}}} & \left( {{Equ}.\mspace{14mu} 1} \right)\end{matrix}$

At the beginning of a complete data recording cycle, the switch 130 a isclosed by a signal 150 and the photodiode 120 is thus depleted. With thefalling edge of the signal 150, at the time nT, a phase 176 of chargeaccumulation of the photodiode 120 begins, wherein the readoutcapacitance 132 a is reset by a signal 152 at the time nT,simultaneously with the falling edge of the signal 150. After resettingthe readout capacitance by the signal 152, the voltage state across thereadout capacitance 132 a containing the contribution of the reset noisecharge and/or kTC noise charge is read out in a non-destructive mannerby means of a signal 154, via the unity-gain amplifier 136 a, andlatched in the CDS stage 140 a which is implemented equivalently to theembodiment of FIG. 1. The following applies for this readout valueU_(FD1,reset):

$\begin{matrix}{U_{{{FD}\; 1},{reset}} = {\frac{Q_{reset}}{C_{FD}} = {U_{DD} + u_{n\; 1}}}} & \left( {{Equ}.\mspace{14mu} 2} \right)\end{matrix}$u_(n1) describing the voltage portion resulting from the reset noise.Simultaneously with the falling edge of the signal 150, at the time nT,transmitting the laser pulse 806 is triggered by the control means 814.It hits the sensor again after the propagation time T_(L) which isproportional to the distance of an object point to the pixel.

In a first phase 156 of charge accumulation of the duration T_(L), onlythe natural background light is detected by the photodiode 120, which iswhy in the phase 156 the voltage across the photodiode capacitance 124(U_(PD)) decreases proportionally to the intensity of the backgroundlight E_(H). Starting from the reflected light pulse impinging at thetime t₁, the change in voltage across the photodiode capacitance 124 ina second phase 160 of charge accumulation is proportional to the sum ofthe intensities of the background radiation E_(H) and the intensityE_(L) of the laser light, which is why in the second phase 160 thevoltage U_(PD) across the photodiode capacitance decreases faster thanin the phase 156. At the end of the charge accumulation phase, theaccumulated photocharge Q_(PD,laser+back) is transferred to the readoutcapacitance 132 a by a signal 162, which again depletes the photodiode120. At the end of the charge transfer, i.e. with the falling edge ofthe signal 162, the following applies for the voltage U_(FD1,signal)across the readout capacitance 132 a:

$\begin{matrix}{U_{{{FD}\; 1},{signal}} = {\frac{Q_{signal}}{C_{FD}} = {U_{DD} + u_{n\; 1} - \underset{\underset{U_{signal}}{︸}}{\frac{Q_{{PD},{{laser} + {back}}}}{C_{FD}}}}}} & {{Equ}.\mspace{14mu} 3}\end{matrix}$

The voltage U_(FD1,signal) is composed of the voltage U_(DD), a noisevoltage portion u_(n1) and a voltage portion U_(signal) resulting fromthe accumulated photocharges, wherein both background light and part ofthe reflected laser light pulse have been used for generating thephotocharge. With the falling edge of the signal 162, another phase 168of charge accumulation of the photodiode 120 is started, wherein thistime the light source 804 does not emit any light pulses.

Simultaneously with the falling edge of the signal 162, the readoutcapacitance 132 b of the second readout branch is reset by a signal 164and, after that, after the end of the reset process, i.e. with thefalling edge of signal 164, the voltage state UFD,2 of the readoutcapacitance 132 b is stored, by a signal 166, in the CDS stage 140 b ofthe second readout branch 128 b, this voltage value containing the noisevoltage portion. In the second phase 168 of charge accumulation, thephotodiode 120 does not receive reflected laser light, so that thevoltage drop U_(PD) across the photodiode capacitance 124 isproportional to the background light intensity E_(H) during the entirephase 168 of charge accumulation. At the end of the accumulation phase,the accumulated photocharge Q_(PD),back is transferred to the readoutcapacitance 132 b of the second readout branch 128 b by a signal 170 sothat with the falling edge of signal 170 the voltage U_(FD2,signal) isacross the readout capacitance 132 b and the following applies:

$\begin{matrix}{U_{{{FD}\; 2},{signal}} = {{U_{DD} + u_{n\; 2} - {\underset{\underset{U_{{signal},{back}}}{︸}}{\frac{Q_{{PD},{back}}}{C_{FD}}}\bigwedge U_{{{FD}\; 2},{reset}}}} = {U_{DD} + u_{n\; 2}}}} & {{Equ}.\mspace{14mu} 4}\end{matrix}$

Eliminating the kTC noise for the individual readout branches 128 a and128 b takes place by calculating the difference in the downstream CDSstages 140 a and 140 b, wherein equation 2 is to be subtracted fromequation 3 for the first readout branch 128 a and the difference ofU_(FD2,signal) and U_(FD2,reset) of equation 4 is calculated for thereadout branch 128 b.

Successive accumulation of N light pulses is assumed for furtherconsideration for correcting the background light portion, wherein thepossibility of multiple accumulation is indicated in the right hatchedpart of FIG. 5. Here, the charge accumulation cycle of the duration T,as described before, is repeated, wherein it should be kept in mind thatthe photodiode 120 has to be depleted before starting a new chargeaccumulation, which may then only take place by a signal 172 via thethird readout branch 128 c in order not to corrupt the signal chargesstored on the readout capacitances 132 a and 132 b. In the furtherrecording cycles of the duration T, resetting the readout capacitances132 a and 132 b by means of the signals 152 and 164 is to be refrainedfrom since the noise voltage portion may only be read out in the firstone of N cycles in order not to erase the signal charges stored on thereadout capacitances 132 a and 132 b.

For further considerations as to correcting the background lightportion, it will subsequently be assumed that a number N of completeacquisition cycles of a duration T have been performed by multipleaccumulation. All in all, the following results as readout signalsU_(FD1,signal) and U_(FD2,signal) of the readout branches 128 a and 128b of FIG. 4, related to a reference voltage U_(ref):

$\begin{matrix}{U_{{{FD}\; 1},{signal}} = {{U_{ref} - {N \cdot {\frac{Q_{{PD},{{laser} + {back}}}}{C_{Fd}}\bigwedge U_{{{FD}\; 2},{signal}}}}} = {U_{ref} - {N \cdot \frac{Q_{{PD},{back}}}{C_{Fd}}}}}} & {{Equ}.\mspace{14mu} 5}\end{matrix}$

Thus, as described before, the portion of reflected laser light iscontained in the signal UFGD1,signal, whereas only the background lightportion is contained in the signal UFD2,Signal.

Calculating the difference of the terms UFD1,signal and UFD2,signaleliminates the background light portion:

$\begin{matrix}{U_{distance} = {N \cdot \frac{Q_{{PD},{laser}}}{C_{FD}}}} & \left( {{Equ}.\mspace{14mu} 6} \right)\end{matrix}$

Calculating the difference usually takes place directly on the CMOSsensor. The difference U_(distance) of equation 6 only carriesnoise-corrected laser light charge information proportional to thedistance. Equation 6 may be converted as follows using the spectralsensitivity S_(spectral) of the photodiode, the received residual energyof the laser signal E_(L) and a length 174 of the laser pulseT_(shutter):

$\begin{matrix}{U_{distance} = {N \cdot \frac{S_{spectral} \cdot E_{L} \cdot \left( {T_{shutter} - T_{L}} \right)}{C_{FD}}}} & \left( {{Equ}.\mspace{14mu} 7} \right)\end{matrix}$

The correction of the background light portion in the readout signalU_(distance) has already taken place at this time, since the readoutresult U_(distance) only contains a portion which depends on lightpropagation time T_(L) and the residual energy of the laser signal E_(L)received at the sensor. However, generally the received residual energyof the laser signal E_(L) will vary from pixel to pixel within thesensor 812, even with equal distances of all pixels of the sensor 812 tothe reflecting object 802, since the reflectance at the surface of theobject 802 to be examined is not spatially constant.

In order to achieve high precision of the readout result, thereflectance variation on the surface of the object 802 has also to becorrected. In order to obtain a readout result U_(distance,ref), anothercomplete readout cycle of the length T including background lightcorrection is performed, wherein the laser signal 174 has to have thesame length like in the first complete accumulation cycle fordetermining the distance-dependent signal U_(distance).

The difference to the first complete accumulation cycle is that thelength of the shutter signal 176 has to be so great that it is ensuredthat each pixel on the sensor 812 will receive the complete laser pulseduring its entire active phase of charge accumulation 176. In otherwords, this means that, on the one hand, the falling edge of the signal150 at the time nT, and thus the beginning of the active chargeaccumulation, for all the pixels has to be temporally before thereflected light pulse 178 arriving, so that a pixel is prevented fromdetecting a signal portion resulting already from laser light, duringthe period of time T_(L), namely the light propagation time.Additionally, the length of the transmitted light pulse 174 has to be atleast of a size so as to ensure that all the pixels of the sensor 812have received the signal of the reflected laser light at the fallingedge of the signal 162, i.e. at the time of the end of the chargeaccumulation phase. Thus, it is ensured that all the pixels detect laserlight during their entire charge accumulation phase of the durationT_(shutter) so that the signal U_(distance,ref) which no longer dependson the light propagation time T_(L) is obtained at the end of a completereadout cycle of the duration T.

$\begin{matrix}{U_{{distance},{ref}} = {N \cdot \frac{S_{spectral} \cdot E_{L} \cdot T_{shutter}}{C_{FD}}}} & \left( {{Equ}.\mspace{14mu} 8} \right)\end{matrix}$

This second pulse sequence of equal laser duration, but considerablylonger shutter time, serves for correcting reflectance variations whichinfluence the amount of reflected laser light E_(L), and the correctionof deviations of the spectral sensitivity. The quotient(U_(distance,ref)/U_(distance)) is calculated for correction purposes,wherein at the same time the light propagation time T_(L) is substitutedby means of introducing the distance d from the object point to therespective pixel and the speed of light c, so that the followingapplies:

$\begin{matrix}{\frac{U_{{distance},{ref}}}{U_{distance}} = \frac{T_{shutter}}{T_{shutter} - \left( \frac{d}{2\; c} \right)}} & \left( {{Equ}.\mspace{14mu} 9} \right)\end{matrix}$

Solving the equation for the distance d of the pixel to the respectiveobject point has the following result:

$\begin{matrix}{d = {2\;{c \cdot T_{shutter} \cdot \left( {1 - \frac{U_{{distance}\;}}{U_{{distance},{ref}}}} \right)}}} & \left( {{Equ}.\mspace{14mu} 10} \right)\end{matrix}$

This shows that, using a pinned photodiode 120 and a respective readoutcircuit according to FIG. 4, a readout result can be obtained in anefficient manner in which, when using a receiving device of FIG. 4 in atwo-dimensional pixel array of a sensor 812 for depth measurements, thereadout signal is completely freed of reset noise portions andbackground light portions, and in which a correction of the variablereflectance of the object 802 to be examined has also been performed.Due to the little noise, large-area sensor matrices including high pixelnumbers may be realized, which, for example in security-relevantapplications, allow reliable distance and depth map images to berecorded.

In another embodiment of the present invention of FIG. 6, two pinnedphotodiodes 182 a and 182 b are used as an independent light-sensitivedetector element and/or as a subpixel of a superpixel including thephotodiodes 182 a and 182 b of a sensor 812 for depth measurements,wherein measuring distances by means of the modulation method presentedin PCT/DE00/03632 is made possible by the arrangement illustrated inFIG. 6.

The basic principle of the method is controlling the phases of chargeaccumulation and respective readout of one or several light-sensitivepixels by means of a modulation signal and synchronizing same with anemitted modulated light signal. Depending on the distance of the sceneobjects, the phase position of the reflected modulation light relativeto the modulation signal changes, the consequence being that thequantity of photoelectrons generated in the pinned photodiode increasesor decreases, depending on the phase difference.

The exemplary implementation of the modulation method by means of thepinned photodiodes 182 a and 182 b in FIG. 6 is to be describedsubsequently referring to FIG. 7 which explains the basic mode offunctioning of the modulation method in greater detail. FIG. 6 shows thesemi-schematic illustration of the pinned photodiodes 182 a and 182 bwhich are electrically separated from each other by a guard structure183 in order to avoid electrical crosstalk. A detailed description ofthe semiconductor structure and the equivalent circuit diagram of thephotodiodes is omitted here since this has already been explained indetail in FIGS. 1 and 2.

Reading out a readout capacitance 184 a from the photodiode 182 a isperformed by a readout device 186 a which is only illustrated hereschematically and exemplarily corresponds to the readout device shown inFIG. 1. A readout capacitance 184 a of the photodiode 182 b mayequivalently be read out by a readout device 186 b. In the embodimentshown, the control means 814 synchronizes the transfer times of thecharge carriers accumulated in the photodiodes 182 a and 182 b to thereadout capacitances belonging to the respective photodiodes by means ofa modulation signal by means of which a switch 188 a and a switch 188 bare opened and closed at times preset by the modulation signal. In thisway, the phase difference between the transmitted and the receivedsignal—and thus the distance information—is contained in the chargequantity transferred to the readout capacitances 184 a and 184 b.

It is explained using FIG. 7 how the control means controls the transfertimes of the charges to the readout capacitances 184 a and 184 b bysignals 190 a and 190 b using a preset modulation signal so that thedesired readout result can be achieved. In FIG. 7, the time t is plottedon the x axis and the figure also shows a light signal 192 modulated bythe control means, which here is a number of square-wave pulses, and thereflected light signal 194 arriving at the sensor 812. Additionally,active phases 196 a and 196 b of the charge accumulation of thephotodiodes 182 a and 182 b are illustrated, as controlled by thecontrol means 814. The charges 198 a and 198 b accumulated on thereadout capacitances 184 a and 184 b are indicated in arbitrary units.The control signal 190 a (TG1) serves for actuating the switch 188 a andthe control signal 190 b (TG2) serves for actuating the switch 188 b,both being generated by the control means and synchronized with thelight signal.

At the beginning of a readout cycle of the duration T, the laser lightpulse 200 is transmitted which arrives at the sensor as a reflectedlight pulse 204 with a phase shift 202 (ΔΦ) to be determined. At thebeginning of the cycle of the duration T, at the time t₀, the photodiode182 a is controlled by the control means 814 such that it will be in aphase of active charge accumulation for a time period 206 up to the endof light pulse emission, i.e. the falling edge of the signal 200. At theend of the accumulation phase 206, at the time t₁, the charge of thephotodiode 182 a is transferred to the readout capacitance 184 a by acontrol signal 208, wherein the charge transfer phase ends with thefalling edge of the signal 208 so that the charge 210 indicated inarbitrary units will be on the readout capacitance 184 a at the time t₁.When light pulse emission ends, at the time t₁, the active chargeaccumulation phase of the photodiode 182 a is stopped by the controlmeans and what begins temporally is a phase 212 of active chargeaccumulation of the photodiode 182 b. At the end of the accumulationcycle of the duration T, at the time t₃, the accumulated charge of thephotodiode 182 b is transferred to the readout capacitance 184 b by thecontrol means by means of the signal 214 so that the charge 216represented in arbitrary units will be on the readout capacitance 184 bat the end of the complete accumulation cycle of the length T.

As can be seen from FIG. 7, the magnitude of the charges 210 and 216accumulated on the readout capacitances depends on the phase shift 202between the transmitted signal and the reflected signal. Evaluatingmeans calculates, after a predetermined number N of successive cycles ofthe duration T, the difference of the accumulated charges 210 and 216which directly contains the information on the phase shift ΔΦ and thusthe distance of the object point to the pixel. In an extreme case of anobject located directly at the sensor, the phase shift 202 would be zeroand thus the accumulated charge 210 in the photodiode 182 a would bemaximum, whereas the accumulated charge 216 in the photodiode 182 b iszero, so that the difference of the accumulated charges 210 and 216 inthis case would be maximum.

In order to illustrate the realization of the method by means of thesemiconductor structure shown in FIG. 6, FIG. 6 also shows the potentialform 220 as results during the transfer phase 208 in the semiconductorstructure, and the potential form 222 as results in the semiconductorstructure during the transfer phase 214. During the transfer phase 208,the switch 188 a is closed so that the charge carriers accumulated inthe photodiode 182 a are transferred to the readout capacitance 184 a bythe potential gradient illustrated in 220. At the same time, the switch188 b is opened and the photodiode 182 b accumulates charges onto itsphotodiode capacitance, without transferring same to the readoutcapacitance 184. The complementary situation 222 results during thetransfer phase 214 when the charge carriers generated are stored in thephotodiode capacitance of the pinned photodiode 182 a, whereas thecharge carriers stored in the photodiode 182 b during the accumulationcycle dissipate via the closed switch 188 b onto the readout capacitance184 b.

An advantage of the modulation method is that the background lightportion is corrected automatically by subtracting the two signalsaccumulated by the different photodiodes and that, in order to increasethe signal-to-background ratio, the integration duration may simply beincreased. An advantage of using a pinned photodiode for realizing amodulation method of FIG. 6 is that no opaque shielding is applied abovethe light-sensitive p⁺-type regions 230 a and 230 b, as is the case forthe CCD gate and/or the photogate mentioned in PCT/DE00/03632. Due tothe fact that the p⁺-type layer of the photodiode is completely exposed,the signal-to-noise ratio and the sensitivity are improved significantlyand, in addition, the modulation method may be realized by means of asensor which is manufactured completely in a cheap standard CMOSprocess.

Although an implementation of the inventive detection device isillustrated referring to FIG. 2 in the p-epi CMOS process, therealization of the pinned photodiode is also possible in thecomplementary technology of an n-epi CMOS process. The CDS stage 32 inFIG. 1 includes the switches Select1_a, Select1_b, the holdingcapacitances 40 a and 40 b, unity-gain amplifiers 38 a and 38 b and thedifference-calculating stage 34 as a possible realization of a CDSstage, wherein any other realization of a CDS stage and/or calculatingthe difference of two analog signals is possible as well. Alternatively,the signals may also be digitalized so that calculating the differencewill take place digitally.

Depleting the photodiode 120 in FIG. 4 during a complete readout cyclemay, if appropriate, be accelerated by closing the switch 130 csimultaneously to closing switch 130 a or 130 b, which is notillustrated explicitly in the wave forms in FIG. 5, but is a possiblealternative.

The type of the modulation signals and cycles determining the time ofcharge transfer onto the readout capacitances 184 a and 184 b in FIG. 6are not defined specifically. They may, like in FIG. 7, be simplesquare-wave signals or they may be sine-shaped or, exemplarily, be apseudo-noise sequence or the like. More complex circuit structures,like, for example, multi-quadrant demultiplexers, and the like, may alsobe realized by means of the pinned photodiode. Furthermore, a distancesensor which is based on the modulation method may be realized by meansof a circuit similar to that of FIG. 4, wherein the modulation signalcontrols the time of reading out the accumulated photocharges of asingle photodiode structure 120 in two independent readout branches 128a and 128 b such that a distance sensor based on the modulation methodmay be realized by means of only one single pinned photodiode.

Depending on the circumstances, the inventive method for detectingoptical radiation may be implemented in either hardware or software. Theimplementation may be on a digital storage medium, in particular on adisc or CD having control signals which may be read out electronicallywhich can cooperate with a programmable computer system such that theinventive method for detecting optical radiation will be executed.Generally, the invention thus also is in a computer program productcomprising a program code stored on a machine-readable carrier forperforming the inventive method when the computer program product runson a computer. In other words, the invention may also be realized as acomputer program comprising a program code for performing the methodwhen the computer program runs on a computer.

While this invention has been described in terms of several embodiments,there are alterations, permutations, and equivalents which fall withinthe scope of this invention. It should also be noted that there are manyalternative ways of implementing the methods and compositions of thepresent invention. It is therefore intended that the following appendedclaims be interpreted as including all such alterations, permutations,and equivalents as fall within the true spirit and scope of the presentinvention.

1. An optical detection device comprising: a photodiode structurecomprising a photodiode capacitance for accumulating charge carriersresponsive to electromagnetic radiation; a readout capacitance; aresetter for resetting the readout capacitance by applying apredetermined voltage to the readout capacitance; a switch forconnecting the photodiode structure to the readout capacitance totransfer the accumulated charge carriers to the readout capacitanceduring a transfer phase and for separating the photodiode structure fromthe readout capacitance during an accumulation phase; a readout unit forreading out the readout capacitance, the readout unit being implementedto read out the readout capacitance for a first time during theaccumulation phase and for a second time after the accumulation phasefor acquiring a first and second readout values and to combine the twovalues for acquiring a readout result; and an additional readoutcapacitance comprising a respective additional resetter for resettingthe additional readout capacitance, and an additional switch forconnecting the photodiode capacitance to the additional readoutcapacitance during an additional transfer phase.
 2. The device of claim1, wherein the resetter is implemented to reset the readout capacitanceat least once during a transfer phase.
 3. The device of claim 1, whereinthe photodiode structure generating the charge carriers includes apinned photodiode.
 4. The device of claim 1, wherein a readout resultdepends on the ratio of the photodiode capacitance and the readoutcapacitance, the quotient of the photodiode capacitance and the readoutcapacitance being positive.
 5. The device of claim 1, wherein thereadout unit is implemented to determine a state of the readoutcapacitance without altering same.
 6. The device of claim 1, wherein theradiation to be measured is pulsed in radiation phases, and wherein thedevice comprises a controller for synchronizing the radiation phaseswith the accumulation and transfer phases.
 7. The device of claim 6,wherein the controller is implemented to synchronize the accumulationphases and the transfer phases with the radiation phases such that thereis at least one pair of an accumulation phase followed by a transferphase which are synchronized with the radiation phase between a firstreset of the readout capacitance and a second reset of the readoutcapacitance.
 8. The device of claim 7, wherein the readout unit isimplemented to read out, at a beginning of the accumulation phase of thefirst pair of accumulation phase and the transfer phase between thefirst and the second reset, the readout capacitance for acquiring afirst readout value, and to read out, at an end of the accumulationphase of the last pair of accumulation phase and transfer phase, thereadout capacitance for acquiring a second readout value, and to outputa difference of the first and second readout values as a readout result.9. The device of claim 1, further comprising: complementary detectiondevice of the same structure as the detection device which includes acomplementary photodiode capacitance, a complementary photodiodestructure, a complementary readout capacitance, a complementaryresetter, a complementary switch and a complementary readout unit;wherein the radiation to be measured is pulsed in radiation phases; acontroller which is implemented to: synchronize the radiation phaseswith the resetter, the switch and the readout unit and with thecomplementary resetter, the complementary switch and the complementaryreadout unit; during a clock cycle of a radiation phase and a phase freefrom radiation, connect and separate the photodiode capacitance and thereadout capacitance twice and, during the clock cycle, actuate theresetter once and acquire two readout results of the readout unit;during a clock cycle, connect and separate the complementary photodiodecapacitance and the complementary readout capacitance twice and actuatethe complementary resetter once and acquire two complementary readoutresults of the complementary readout unit.
 10. A system for 3Dmeasurements of objects, comprising: a pulsed light source; a pluralityof optical detection device comprising: a photodiode structurecomprising a photodiode capacitance for accumulating charge carriersresponsive to electromagnetic radiation; a readout capacitance; aresetter for resetting the readout capacitance by applying apredetermined voltage to the readout capacitance; a switch forconnecting the photodiode structure to the readout capacitance totransfer the accumulated charge carriers to the readout capacitanceduring a transfer phase and for separating the photodiode structure fromthe readout capacitance during an accumulation phase; a readout unit forreading out the readout capacitance, the readout unit being implementedto read out the readout capacitance for a first time during theaccumulation phase and for a second time after the accumulation phasefor acquiring a first and second readout values and to combine the twovalues for acquiring a readout result; and an additional readoutcapacitance comprising a respective additional resetter for resettingthe additional readout capacitance, and an additional switch forconnecting the photodiode capacitance to the additional readoutcapacitance during an additional transfer phase, the detection units ofwhich are arranged in a matrix, wherein the accumulation phase and thetransfer phase are synchronized with a radiation phase by means of theswitch; an imager for imaging an object onto the matrix; and anevaluator for generating 3D information concerning the object based onthe states of the readout capacitances of the plurality of devices. 11.A method for operating an optical detection device including aphotodiode structure comprising a photodiode capacitance foraccumulating charge carriers responsive to electromagnetic radiation anda readout capacitance, and an additional readout capacitance,comprising: resetting the readout capacitance by applying apredetermined voltage to the readout capacitance by means of a resetter;connecting the photodiode structure to the readout capacitance by aswitch to transfer the accumulated charge carriers to the readoutcapacitance during a transfer phase and separate the photodiodestructure from the readout capacitance during an accumulation phase;reading out the readout capacitance by a readout unit, the readout unitbeing implemented to read out the readout capacitance for a first timeduring the accumulation phase and for a second time after theaccumulation phase for acquiring a first and second readout values, andto combine the two values for acquiring a readout result; resetting theadditional capacitance; connecting the photodiode structure to theadditional readout capacitance by an additional switch to transfer thesecond accumulated charge carriers to the additional readout capacitanceduring a transfer phase and separate the photodiode structure from theadditional readout capacitance during an accumulation phase; and readingout the additional readout capacitance to read out the additionalreadout capacitance for a first time during the accumulation phase andfor a second time after the accumulation phase for acquiring a first andsecond readout values, and to combine the two value for acquiring anadditional readout result.
 12. A non-transitory, computer readablemedium storing a computer program, when run on a computer, the computerprogram performs a method for operating an optical detection deviceincluding a photodiode structure comprising a photodiode capacitance foraccumulating charge carriers responsive to electromagnetic radiation anda readout capacitance, and an additional readout capacitance, the methodof the computer program stored on the non-transitory, computer readablemedium comprising: resetting the readout capacitance by applying apredetermined voltage to the readout capacitance by means of a resetter;connecting the photodiode structure to the readout capacitance by aswitch to transfer the accumulated charge carriers to the readoutcapacitance during a transfer phase and separate the photodiodestructure from the readout capacitance during an accumulation phase;reading out the readout capacitance by a readout unit, the readout unitbeing implemented to read out the readout capacitance for a first timeduring the accumulation phase and for a second time after theaccumulation phase for acquiring a first and second readout values, andto combine the two values for acquiring a readout result; resetting theadditional capacitance; connecting the photodiode structure to theadditional readout capacitance by an additional switch to transfer thesecond accumulated charge carriers to the additional readout capacitanceduring a transfer phase and separate the photodiode structure from theadditional readout capacitance during an accumulation phase; and readingout the additional readout capacitance to read out the additionalreadout capacitance for a first time during the accumulation phase andfor a second time after the accumulation phase for acquiring a first andsecond readout values, and to combine the two value for acquiring anadditional readout result.